Minimally invasive devices, such as catheters, are often employed for medical procedures, including those involving mapping, ablation, dilation, and the like. For example, a thermal diagnostic or treatment procedure may involve permanently and/or temporarily exchanging thermal energy with a targeted tissue region, such as creating a series of inter-connecting or otherwise substantially continuous lesions in order to electrically isolate tissue believed to be the source of an arrhythmia. An example of a thermal mechanism for diagnosis and treatment is a cryogenic device that uses the energy transfer derived from thermodynamic changes occurring in the flow of a cryogen therethrough to create a net transfer of heat flow from the target tissue to the device. The quality and magnitude of heat transfer is regulated in large part by the device configuration and control of the cryogen flow regime within the device.
Structurally, cooling can be achieved through injection of high pressure refrigerant through an orifice. Upon injection from the orifice, a refrigerant may undergo two primary thermodynamic changes: (i) expanding to low pressure and temperature through positive Joule-Thomson throttling, and (ii) undergoing a phase change from liquid to vapor, thereby absorbing heat of vaporization. The resultant flow of low temperature refrigerant through the device acts to absorb heat from the target tissue and thereby cool the tissue to the desired temperature.
The efficacy of a thermal exchange procedure may be substantially affected by the fluid flow through the device as well as the thermal conductivity between a device and the tissue site. To provide shorter treatment durations and increased efficacy for the particular treatment provided, it is desirable to optimize the heat transfer between the specific tissue to be treated and the cryogenic element or device. Such optimization may include providing accurate and precise fluid delivery through a selected device to achieve the desired thermal affect in the physiological location being treated. Such physiological locations often include numerous environmental factors that can constantly change, resulting in fluctuating thermal conditions arising between the tissue and the device. For example, blood or other body fluids passing through the vicinity of the thermal device can reduce the quality of thermal exchange with the targeted tissue, which can then necessitate additional “cooling power” or refrigerant flow in the case of cryogenic treatments in order to complete the desired treatment.
Accordingly, it is desirable to provide systems and methods of use thereof that provide accurate and precise control over fluid delivery to and through such devices in order to optimize the efficacy of the device in a physiological environment.